The present invention relates to a light-receiving element array device comprising a rectangular light-receiving element array chip incorporated in a package, and more specifically to a light-receiving element array device in which a distance from a center of a light-receiving section of the light-receiving element array chip to a longer edge of the rectangular package is small. Further this invention relates to an optical demultiplexer using the light-receiving element array device as described above.
The optical demultiplexer is used, for instance, in the photoelectric communications based on the wavelength multiplexed transmission system as a device for separating light transferred to the receiving side in the multiplexed form to several light components each corresponding to a wavelength. Optical demultiplexers having various configurations have been developed, and one of the representative optical demultiplexers uses therein a diffraction grating as an optical demultiplexing element.
One of the optical demultiplexers using a diffraction filter therein has the configuration generally called as xe2x80x9cLittrow type arrangementxe2x80x9d. This type of optical demultiplexer comprises an input optical fiber, a collimator lens, and a diffraction grating, and in this optical demultiplexer, an optical signal from the input optical fiber is collimated by the collimator lens and is guided to the diffraction grating, and the diffracted light is again converged by the collimator lens for the light to be detected. For detection of the light, the diffracted light is guided to a light detector using a number of optical fibers or a light guide path array.
As another type of optical demultiplexer using a diffraction grating therein, there is the one comprising an input optical fiber, a collimator lens, a reflection mirror, and a diffraction grating, and in this type of optical demultiplexer, an optical signal from the input optical fiber is collimated by the collimator lens, reflected by the reflection mirror, and guided to the diffraction grating, and then the diffracted light is guided to the light detector.
The light-receiving element array device is used as a light detector for the optical demultiplexer as described above. The light-receiving element array device has the configuration in which a rectangular chip having a light-receiving section with a number of light-receiving elements arranged in the array form thereon is packaged at a center of a rectangular DIP type of package having a number of external leads and further bonding pads for the chip and bonding terminals each at an inner edge of the corresponding external lead are connected to each other with a bonding wire. The external leads are led out from both edges of the package. All of the light-receiving element array devices commercially available at present in the market use the DIP type of package as described above.
Of the optical demultiplexers as described above, in the latter type of optical demultiplexer using a reflection mirror therein, an optical axis of the light branched by the diffraction grating is substantially vertical to an optical axis of the light induced onto the reflection mirror, so that substantial size reduction is difficult.
In contrast, in the former optical demultiplexer based on the xe2x80x9cLittrow type arrangementxe2x80x9d, an optical axis of the light passing through the collimator lens and induced onto the diffraction grating is substantially identical to that of the diffracted light passing through and coming out from the collimator lens, so that the substantial size reduction is possible.
When the light-receiving element array device having the conventional configuration is used as a light detector, however, as the light-receiving section can not be arranged at a position adjacent to an input optical fiber, coma aberration of the collimator lens increases, which results in degradation of the optical characteristics of the light-receiving element array device. Namely the distance from the light-receiving section of the light-receiving element array chip to a longer edge of the package is large, and the distance between the light-receiving section and the input optical fiber can not be shortened due to the package""s size, although it is necessary to arrange an input optical fiber and a light-receiving section at conjugate positions. Therefore a length of the optical system corresponding to the distance between the input optical fiber and the light-receiving section is required, which is a large obstacle in size reduction of an optical demultiplexer.
It is an object of the present invention to provide a light-receiving element array device having the construction in which the distance between a longer edge of a rectangular package and a center of a light-receiving section of a light-receiving element array chip is small.
It is another object of the present invention to provide a light-receiving element array device well adapted to use as an optical demultiplexer based on the Littrow type of arrangement in which the light-receiving section can be arranged at a position adjacent to the input optical fiber.
It is still another object of the present invention to provide an optical demultiplexer based on the Littrow type of arrangement which enables suppression of coma aberration and reduction of the optical system length even when a light-receiving element array device is used therein as a light detector.
The light-receiving element array device according to the present invention is based on the configuration in which a light-receiving element array chip having a light-receiving section with a number of light-receiving elements arranged thereon in the array form is sealed in a rectangular package having a number of external leads and bonding terminals connected to the external leads respectively and each of the bonding terminals of the package is connected to a bonding pad on the chip with a bonding wire or the like. The light-receiving element array device according to the present invention may be based on any of the following constructions:
(1) in which no bonding pad is provided in the side along one longer edge of the light-receiving element array chip and the chip is accommodated in the package at a position displaced to one side thereof so that the longer edge of the chip, along which no bonding pad is provided, is close to one longer edge of the package,
(2) in which no bonding terminal is provided in the side along one longer edge of the package and the chip is accommodated in the package at a position displaced to one side thereof so that a longer edge of the chip is close to the longer edge of the package along which no bonding terminal is provided, or
(3) in which no external lead is provided along one longer edge of the package and the chip is accommodated in the package at a position displaced to one side thereof so that a longer edge of the chip is close to the longer edge of the package along which no bonding terminal is provided, or on a combination of the constructions. More specifically it is preferable that the distance between a center of the light-receiving section of the chip and a longer edge of the package is 3 mm or less when any of the constructions described above is employed.
The optical demultiplexer according to the present invention is based on the configuration in which an input optical fiber, a collimator lens, and a light-receiving element array are arranged according to the Littrow type arrangement. With this arrangement, the condition of W/Lxe2x89xa64/50 must be satisfied, wherein L (mm) indicates the distance from a main surface position of the collimator lens to the light-receiving section of the light-receiving element array and W (mm) indicates the distance from a center of the light-receiving section of the chip to a center of the input optical fiber. To satisfy this condition, the light-receiving element array device in which the chip is accommodated in the package at a position displaced to one side thereof as described above is used, and the light-receiving element array device is arranged so that the light-receiving section is close to and faces the input optical fiber mounting section.